The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Magnetoresistive (MR) read sensors, commonly referred to as MR heads, are used in all high capacity disk drives. An MR sensor detects a magnetic field through the change in its resistance of as a function of the strength and direction of the magnetic flux being sensed by the MR layer. The standard type of MR sensor in today's disk drives employs the giant magnetoresistive (GMR) effect, such that the resistance varies as a function of the spin-dependent transmission of the conduction electrons between two or more ferromagnetic layers separated by a non-magnetic spacer layers, and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers. The resistance of these sensors depends on the relative orientation of the magnetization of the different magnetic layers.
GMR sensors whose resistance depends primarily on the relative magnetization of only two layers of ferromagnetic material (e.g., Ni—Fe) separated by a layer of non-magnetic material (e.g., Cu) are generally referred to as spin valve (SV) sensors. In a “simple”, SV sensor, one of the ferromagnetic layers, referred to as the reference layer (or pinned layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., PtMn) layer. The pinning field generated by the antiferromagnetic layer should be sufficiently large to ensure that the magnetization direction of the reference layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a “simple” SV sensor operating on the basis of the GMR effect.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. FIG. 1A shows a prior art SV sensor 100 comprising a free layer (free ferromagnetic layer) 110 separated from a reference layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the reference layer 120 is fixed by exchange pinning to an antiferromagnetic (AFM) layer 130.
FIG. 1B shows a perspective view of the SV sensor 100 of FIG. 1A.
Almost universally employed in present day SV sensors is the use is the use of antiparallel (AP)-pinning. In such AP-pinned SV sensors, the reference layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic AP-coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The first ferromagnetic layer, referred to as the pinned layer, has its magnetization pinned/fixed in orientation by direct exchange coupling to an AFM layer. The second ferromagnetic layer serves as the reference layer in determining the resistance of the device, is strongly AP-coupled to the pinned layer, and by effect is also fixed in orientation. The cancellation of magnetic moment and demagnetizing fields of the AP-aligned pinned and reference layers greatly improves the stability of the reference layer relative to that obtained for the simple SV sensor of FIG. 1A.
Referring to FIG. 1C, an AP-Pinned SV sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic (pinned) layer 226 and a second ferromagnetic (reference) layer 222 separated by an antiparallel coupling layer (APC) 224 of nonmagnetic material. The two ferromagnetic layers 226, 222 (FM1 and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
Variations of the resistance R of the spin valve sensor are known to be a linear function of Q=cos(θ), where θ is the angle between the (in-plane) magnetization vectors of the reference and free layer structures. Specifically, θ≡θf−θr, where θf is the angle of (in-plane) magnetization of the free layer and θr represents the angle of (in-plane) magnetization of the reference layer. The sensitivity of the sensor is quantified by its magnetoresistive coefficient ΔR/R0, where ΔR=Rmax−Rmin the maximum change in the resistance of the sensor, and R0=R(Q=0) is the resistance of the sensor when the magnetic moments are orthogonal. It is virtually always the case in SV sensors as practiced in the art that Rmax=R(Q=−1)=R(θ=180°) and Rmin=R(Q=+1)=R(θ=0).
In operation, the SV sensor is subjected to positive and negative magnetic signal fields Hsig from a moving magnetic disk. These positive and negative signal fields are typically equal in magnitude, and oriented orthogonal to the plane of the disk (or ABS plane). It is desirable that positive and negative changes in the resistance of the spin valve read head be equal so that the positive and negative readback signals are equal.
When the direction of the magnetic moment of the pinned/reference layer structures are exchange pinned perpendicular to the ABS, i.e., such that θr=±90°, then Q=±sin(θf), where θf is measured from an axis mutually parallel to the planes of the free layer and the ABS. Since R(Q) is linear with variation of Q, it follows that R0=R(Q=0)=R(θf=0) will be exactly midway between Rmax=R(Q=−1) and Rmin=R(Q=+1).
It is further well known magnetically that the rotation of the free layer magnetization angle θf in response to magnetic signal fields from the disk, is such that sin(θf) will vary approximately linearly with the amplitude of the signal field Hsig. This is particularly true if θfb=0 is approximately the (quiescent) bias-point orientation of the free layer in the absence of signal fields, in which case the sensitivity d(sin(θf)/dHsig is also generally maximized. It follows that the optimum bias point configuration: θr≅±90°, θfb=0° will concurrently maximize both the small signal sensitivity and linear dynamic range SV sensor, while simultaneously minimizing the asymmetry in resistance change R(Hsig)−R0 with respect to the polarity of signal field Hsig. These design considerations for optimized SV sensor performance are illustrated in FIGS. 2A-2B, and are well known in the prior and present day art for CIP sensors.
The location on the transfer curve of the actual bias point is influenced by several major factors. These include demagnetizing fields, both intralayer (shape anisotropy), and mutual interlayer dipolar coupling, combined with interfacial coupling fields between the free and reference/pin layers, the pinning field strength Hpin exerted on the pinned layer by exchange coupling to the AFM to maintain θr≅±90°, and, a longitudinal bias field HLB (typically supplied by layers of permanent, hard magnet material contiguous to the track edges of the SV sensor), to stabilize the free layer magnetization in a quiescent/bias state with θfb≅0° Given the inevitable anisotropic stress relief at the lapped, ABS edge of the sensor, stress-induced magnetic anisotropy due to nonzero magnetostriction in any of these magnetic layers may also play a role.
The above descriptions of SV sensors apply primarily to what are known in the art as Current-in-Plane (CIP) spin-valve sensors (CIP-SV), in which the bias current flows primarily in the plane of all constituent layers of the SV structure. CIP-SV sensors have been at the core of HDD read head technology for over one decade. For all such devices, the bias current is injected into the sensor through metallic conductive layers (which may include the aforementioned hard bias films) which form a junction with the sensor at the edges which define its physical track-width (TT). Like the sensor itself, the metallic lead structures must be contained within, and maintained electrically insulated from, the two (lower and upper) magnetic conductive shields (typically NiFe) whose gap length G primarily determines the linear (downtrack) resolution capability of the sensor.
As HDD product areal densities exceed 100 Gbit/sq. in., the size of G, TW, and sensor stripe height SH (with SH≈TW typically) are all necessarily being decreased to below 100 nm size. These small SH, combined with constraints on the thickness of the metallic leads at/near the junctions in order to fit within the shield gap G. result in large parasitic junction resistance Rpar which scales roughly as 1/SH. At the 100 nm device size, Rpar is already comparable to the intrinsic sensor resistance Rint=Rsq×(TW/SH), which for fixed sensor sheet resistance Rsq and a fixed, optimized aspect ratio SH/TW≈1, is scale invariant. Because the bias voltage limitations due to Joule heating, the sensor output will be limited by the ratio of Rint/(Rint+Rpar) which scales unfavorably as sensor size is reduced. Due to this, along with fabrication difficulty associated in lithographic limitations in forming the electrical junctions and maintaining robust electrical insulation from the shields, it is projected that CIP-SV sensor technology will not be extendable to sensor sizes at/near or below roughly 50 nm.
It is in this future, but approaching 50 nm sensor size regime, where Current-Perpendicular-to-Plane (CPP) technology is expected to replace the CIP devices used in virtually all disk-drive products to date. For CPP, the shields can serve as the leads, obviating the need for electrical isolation, and essentially eliminating parasitic resistance problems. The two prime candidates for CPP sensors used tunneling magnetoresistance (TMR), or CPP-GMR, the latter being most similar to in physical mechanism to CIP-GMR devices. Of these two, TMR sensors have to date received the most immediate attention and development for the first generation of CPP read heads, due to their potentially very large ΔR/R0>>10% However, the large resistance area product (RA) of TMR tunnel barriers (˜2 Ω-μm2) would predict prohibitively large device impedances RA/(TW×SH) in the ˜50 nm device size regime. For this reason, CPP GMR-SV sensors (CPP-SV) with (RA≈0.05-0.2 Ω-μm2) are receiving attention for longer term R&D as future read head technology.
Although similar, the transport properties of the CPP-SV differ somewhat from that of the CIP-SV. ,In particular, the magnetoresistance R(Q)−R0 varies nonlinearly in the quantity Q=cos(θ) described previously. FIGS. 2A-B qualitatively illustrates the change of resistance R as a function of cos(θ) for a CPP-SV device, the actual layer structure being similar to a CIP-SV. It is believed that this aspect of CPP-SV and its consequences on bias point optimization has heretofore not been fully appreciated to the magnetic head community.
The inventors have found that this nonlinearity is such as to shift the optimum bias point to have non-orthogonal orientation between free and reference layer magnetization, such that Qb=cos(θfb−θr) is negative rather than zero, perhaps as large as Qb≈−0.5. The present invention addresses several different ways to achieve this non-orthogonal bias point.